The roles of biosynthesis and biodegradation in establishing the intracellular concentration of certain glycolytic enzymes in healthy and neoplastic conditions are being investigated. Also the factors regulating the rates of these processes are being explored. Presently rate constants for synthesis and degradation of lactate dehydrogenase isozymes are being determined in mammary tissue of Fischer rats during pregnancy, lactation, and involution and in the 3230 AC transplantable Fischer rat mammary tumor.